Polyurethane elastic fibers obtained from 4,4'-diphenylmethane diisocyanate, polyhydroxy polymer with a relatively low degree of polymerization, multifunctional active hydrogen compounds, and so on exhibit high rubber elasticity, superior mechanical properties in tensile stress and recoverability, and excellent thermal property. For this reason, they have been given much attention and used as functional materials for clothes such as foundation garments, socks, sportswears, and so on.
However, it has been known that exposure of goods made with elastic fibers which have been formed mainly from long chain synthetic elastic segmented polyurethanes to chlorinated aqueous environments with chlorine bleaching agents can cause considerable lowering of the physical properties of the segmented polyurethane. It has been also known that swimwear made with polyurethane fibers and polyamide fibers is subject to lowered physical properties of the fibers upon long-term exposure to the water in swimming pools containing 0.5-3 ppm (parts per million) active chlorine.
In fact, many attempts have been made so far to impart proof or resistance to chlorine-induced degradation. For example, U.S. Pat. No. 4,340,527 teaches zinc oxide, and Japanese Patent Publication No. 35283/1986 teaches magnesium oxide and alminium oxide as additives which prevent chlorine-induced degradation.
Nevertheless, improvements are still needed since the above-mentioned polyurethane elastic fiber containing a chlorine-induced degradation inhibitor, which is used to manufacture union knit fabric loses most of the resistance to chlorine after dyeing, etc., because the degradation inhibitor once contained in the fiber elutes out during dyeing, finishing and processing stages, particularly during the dyeing process which the goods made of the union knit fabric undergo, due to a low pH of dye liquor despite the resistance to chlorine which the raw fiber possesses.